44
votes
Accepted
Does the Earth constantly lose mass?
You are wrong that "to keep any object in circular motion requires energy"
In a circular orbit, the force of gravity is always perpendicular to the motion of the moon, so no work is done by ...
- 115k
43
votes
What is the shape of orbit assuming gravity does not depend on distance?
Circular orbits are always possible for any central force law, but noncircular orbits would resemble rosettes. Here's a specific example for the case where the force is constant with distance:
By ...
- 4,260
28
votes
Accepted
How would "dark matter", subject only to gravity, behave?
What you describe is the standard paradigm in cosmological physics, so it has been studied extensively. The basic consequence of dark matter not having significant nongravitational interactions is ...
- 4,260
22
votes
If the Earth had another moon would it be better protected from asteroids?
I don't quite buy JamesK's claim that "the moon only covers less than 0.001% of the sky, and so leaves us vulnerable to 99.999% of asteroids."
That argument would work if typical asteroids 1)...
- 2,506
16
votes
Does the escape velocity formula take into account how a gravitationally bound object's distance to its primary increases before coming back down?
The escape speed is defined in Newtonian physics simply by demanding that the sum of kinetic energy at launch (ballistically, with no power applied thereafter) and gravitational potential energy at ...
- 146k
16
votes
Could the human body feel the sudden disappearance or end of a gravitational force?
Firstly, the sun can't just "disappear". Even if it were converted by magic into "pure energy", that energy can't go anywhere faster than the speed of light, and Energy has ...
- 115k
13
votes
Accepted
Could we detect dark matter by black holes gaining unexplained mass?
The accretion rate is far too small to make much difference to Galactic black holes, but how could this be distinguished from the accretion of normal, baryonic matter in any case?
In fact it is easier ...
- 146k
12
votes
Concerning a binary system of stars/planets/black holes could one of them be ejected before eventually merging or colliding?
Not in Newtonian gravity with particles. This situation is soluble with stable elliptical orbits, so any examples would have to depend on either Relativity, or that the bodies are not particles.
If ...
- 115k
12
votes
Does the escape velocity formula take into account how a gravitationally bound object's distance to its primary increases before coming back down?
Yes, escape speed is an instantaneous calculation at the distance 'r' from the center of the object, as that changes you have to recalculate your escape speed.
For example, these are the escape ...
- 851
11
votes
If the Earth had another moon would it be better protected from asteroids?
Neither effect is significant.
Asteroids hit the Earth because they happen to be on a collision course with the Earth. The Earth's gravity can deflect some that might just miss the Earth onto a ...
- 115k
8
votes
What is the shape of orbit assuming gravity does not depend on distance?
In addition to Sten's great answer, it should be noted that under constant gravity all orbits are bounded. Therefore, there aren't any orbit like the hyperbolic ones mentioned in the question.
That ...
- 1,720
8
votes
Accepted
Is the speed of time much slower on sun surface?
Escape velocity from the surface of the Sun is 617.6 km/sec, or about 0.00206 times the speed of light. Denoting this ratio (escape velocity / speed of light) as $\beta$, time dilation at the surface ...
- 33.4k
8
votes
Accepted
Celestial "orbits"
Physical theories describe how things change in given circumstances (if the theory is right). In practice this means that they are applied in simplified ways, where the simplifications of ...
- 14k
7
votes
Does the Earth constantly lose mass?
Earth does not lose mass to the moon's orbit. However, there are a few ways it can lose energy to gravitational effects. One way it can lose energy is to gravitational slingshotting, from satellites, ...
- 71
7
votes
Accepted
Are there really confined Globular Clusters?
You are asking for a star cluster that sits at the bottom of an infinite potential well. Such clusters do not exist because they are largely the source of the potential that they reside in.
There are ...
- 146k
6
votes
Accepted
Dark Matter's effect on galaxy structure
The dark matter isn't directly responsible for the rotation of the Galaxy - that is a consequence of the initial angular momentum (or the angular momentum it has accrued during its formation).
Dark ...
- 146k
6
votes
Accepted
If you are in a deep gravity well, where time goes by more slowly, do you see the unfolding of a cosmic event at a different rate?
Time dilation is related to differences in gravitational potential in General Relativity.
Observing a clock situated deep in a potential well, a distant observer would see it running slow. Vice-versa, ...
- 146k
6
votes
Can black holes even exist [if mass cannot be retained near the collapse threshold]?
A different way of looking at the issue: what properties would matter need to have to avoid imploding into a black hole if enough matter was gathered in a static sphere?
The key thing about black ...
- 14k
5
votes
Is it possible to detect gravitational lensing of both light and gravitational waves originating from the same event?
Yes; typically, if the light is lensed, the gravitational waves are also lensed, and vice versa.
The main exception is if the lens is small. The current observatories, LIGO/Virgo/KAGRA, are sensitive ...
- 4,260
5
votes
Accepted
Farthest distance two objects are "gravitationally bounded", considering expansion of the Universe
Gravitational boundness is a tricky question. It's essentially asking, "will these two objects come together at any point in the future?" However, we can give approximate answers.
If there ...
- 4,260
5
votes
Eccentricity with a law of gravity different from the classical one
A planet following this law would either:
Follow a circular orbit. In this case the value of R would be constant and so the magnitude of the central force would also be constant, only the direction ...
- 115k
5
votes
Accepted
How to turn find velocity dispersion from radial velocity
The key assumption is that orbital velocity vectors are distributed randomly. Then the dispersion of velocities along each axis must be the same, e.g. $\sigma_x=\sigma_y=\sigma_z$ if $x$, $y$, and $z$ ...
- 4,260
5
votes
Accepted
What is everything wrong with this theory of dark matter?
Sorry, but this is not correct. In the diagram, spacetime and hence the universe is the curved line. The sun, Earth, and any dark matter are not on spacetime, they are in it.
Visualising spacetime as ...
- 115k
5
votes
Accepted
Spiral Galaxies
You're right that a central mass can't explain the spiral structure (and you're also right about the tangent motion in case the string breaks). Spiral structure is indeed partially caused by gravity, ...
- 37k
5
votes
Celestial "orbits"
You are right. But you miss a few quirks of reality, or some fine-print of the applicable physics.
Yes, the Moon is moving away from Earth as slowing down Earth's spin rotation means that angular ...
- 17.3k
4
votes
How light could the lightest objects able to exist in a stable orbit be?
Here's one perspective. Newtonian gravity is symmetrical under two kinds of rescaling:
If you scale lengths and velocities by $\lambda$ and masses by $\lambda^3$, the system will evolve in exactly ...
- 4,260
4
votes
Accepted
Does a planets mass affect its gravitational pull? Let's say earth increased or decreased in mass could that theoretically affect gravity?
No, gravity is special, because it's pull is related to mass, but acceleration for a given force is inversely proportional to mass.
Galileo noticed this: he observed that two masses, one small and ...
- 115k
4
votes
Accepted
Lack of objects between heliopause and Oort cloud?
There is no "gap" other than an apparent one caused by the use of logarithmic axes.
The density of Oort cloud objects is thought to decrease with increasing distance from the Sun. However, ...
- 146k
4
votes
What is the shape of orbit assuming gravity does not depend on distance?
There wouldn't be standard orbits at all.
The Earth orbits the Sun because the Sun's mass dominates the local spacetime. By removing distance from the gravitational equation, the Earth would, with ...
4
votes
Concerning a binary system of stars/planets/black holes could one of them be ejected before eventually merging or colliding?
When a binary star goes supernova, not only can they get separated, but one of them or both may be even ejected from the galaxy they are in. See https://www.nasa.gov/mission_pages/chandra/images/...
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